Card number 4 allows memories to be locked away in the person's mind or be slightly altered to change a few details within the memory, whereas card number 18 allows the holder to 'enter' the person's mind and physically look through their memories. As for memories locked by card number 4, card number 18 would be able to unlock them if a certain amount of emotional strength is supplied and depending on how long they've been locked away/how strong the memory is. Essentially they'd be freeing the memory from the chains that are holding it down which requires a lot of power if the chains are strong. Then once the chains are broken, the memory will return in the person's mind and The Moon card will be able to look at it in the same way as usual. Hope that makes sense :)